To study the evolution rule of the Brewster effect with the parameters in complicated ground circumstance, the geometrical models of composite rough surfaces with diverse structures are established based on the composite rough surface modeling theory. And the Brewster effects of diverse composite rough surfaces are researched using an adaptive iterative physical optics based on facets of complicated composite rough surfaces. The results show that the dielectric constant and the roughness are significant factors which determine the position of the Brewster angle. The positions of the Brewter angles for different dielectric constants are distinct. The Brewster angle increases with the increase of the root mean square height while it decreases with the increase of the correlation length. Both the change of the regional dielectric constant and that of the regional roughness can lead to the deviation of the Brewster angle which is defined as the Brewster fusion effect, meanwhile the Brewster fusion effect is more remarkable induced by variance of roughness in rough surface region whose dielectric constant is dominant. Therefore, when the dielectric constant and the roughness of the complicated ground circumstance are known, the Brewster angle can be determined by considering the Brewster fusion effect, which can provide a reliable theoretical basis for the detection of super-low altitude targets in the complicated circumstances.